Syndiotactic 1,2-polybutadiene can be made by solution, emulsion or suspension polymerization. The syndiotactic 1,2-polybutadiene from solution, emulsion, or suspension polymerization typically has a melting temperature which is within the range of about 195.degree. C. to 215.degree. C.
Various transition metal catalyst systems based on cobalt, titanium, vanadium, chromium, and molybdenum have been reported in the prior art for the preparation of syndiotactic 1,2-polybutadiene (see, e.g., L. Porri and A. Giarrusso, in Comprehensive Polymer Science, edited by G. C. Eastmond, A. Ledwith, S. Russo and P. Sigwalt, Pergamon Press: Oxford, 1989, Volume 4, Page 53). However, the majority of these catalyst systems have no practical utility because they have low catalytic activity or poor stereoselectivity and in some cases produce low molecular weight polymers or crosslinked polymers unsuitable for commercial use.
The following cobalt-based catalyst systems are well known for the preparation of syndiotactic 1,2-polybutadiene: (1) cobalt bis(acetylacetonate)/triethyl aluminum/water/triphenyl phosphine (U.S. Pat. Nos. 3,498,963 and 4,182,813; Jap. Kokoku 44-32426, assigned to Japan Synthetic Rubber Co. Ltd.), and (2) cobalt tris(acetylacetonate)/triethyl aluminum/carbon disulfide (U.S. Pat. No. 3,778,424; Jap. Kokoku 72-19,892, 81-18,127, 74-17,666, and 74-17,667; Jap. Kokai 81-88,408, 81-88,409, 81-88,410, 75-59,480, 75-121,380, and 75-121,379, assigned to Ube Industries Ltd.). These two catalyst systems also have serious disadvantages.
The cobalt bis(acetylacetonate)/triethyl aluminum/water/triphenyl phosphine system yields syndiotactic 1,2-polybutadiene having very low crystallinity. In addition, this catalyst system develops sufficient catalytic activity only in halogenated hydrocarbon solvents as the polymerization medium, and halogenated solvents present the problems of toxicity.
The cobalt tris(acetylacetonate)/triethyl aluminum/carbon disulfide system uses carbon disulfide as one of the catalyst components. Because of its high volatility, obnoxious smell, low flash point as well as toxicity, carbon disulfide is difficult and dangerous to use and requires expensive safety measures to prevent even minimal amounts escaping into the atmosphere. Furthermore, the syndiotactic 1,2-polybutadiene produced with this catalyst system has a very high melting temperature within the range of 200-210.degree. C., which makes it difficult to process the polymer. Although the melting temperature of the syndiotactic 1,2-polybutadiene can be reduced by the use of a catalyst modifier as a fourth catalyst component, the presence of such a catalyst modifier also has an adverse effect on the catalyst activity and polymer yields. Accordingly, many restrictions are required for the industrial utilization of the two aforesaid cobalt-based catalyst systems of the prior art.
Coordination catalyst systems based on chromium-containing compounds such as chromium(III) acetylacetonate/triethylaluminum have been known for a long time, but they have low catalytic activity and poor stereoselectivity for the polymerization of 1,3-butadiene and give rise to oligomers, low molecular weight liquid polymers or crosslinked polymers. Therefore, these chromium-based catalyst systems of the prior art have no industrial utility.
Japanese patents JP-A-7306939 and JP-A-7364178, both assigned to Mitsubishi, disclose a process for polymerization of 1,3-butadiene to amorphous 1,2-polybutadiene by using a ternary catalyst system comprising: (a) a soluble chromium(III) compound, (b) a trialkyl aluminum compound, and (c) a dialkyl hydrogen phosphite. The product was reported to be a white rubbery polymer which contained a portion of gel and displayed no obvious melting point.
U.S. Pat. No. 4,751,275, assigned to Bayer, discloses a process for the preparation of syndiotactic 1,2-polybutadiene by solution polymerization of 1,3-butadiene in a hydrocarbon polymerization medium. The catalyst system used in this solution polymerization contains a hydrocarbon-soluble chromium(III) compound, a trialkylaluminum compound, and dineopentyl hydrogen phosphite or methyl neopentyl hydrogen phosphite. However, the polymerization product was not well characterized as neither the melting temperature nor the degree of syndiotacticity of the product is reported.
U.S. Pat. No. 4,168,357 and U.S. Pat. No. 4,168,374, both assigned to Goodyear, describe a similar chromium-based catalyst system for the preparation of high cis-1,4-polypentadiene. They disclose dialkylaluminum hydrides for preparing high cis-1,4-polypentadiene, e.g. in column 2, lines 44-52 of U.S. Pat. No. 4,168,357, but they do not claim dialkylaluminum hydrides as part of their catalyst system in the claims.